Anthropologists split on what fossil find means about early humans

Published 4:00 am, Thursday, August 9, 2007

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Frederick Kyalo Manthi , Phd, holds the H. erectus complete skull he discovered in 2000 near lake Turkana in Kenya, Wednesday, Aug. 8, 2007 at the National Museum of Kenya in Nairobi. Surprising fossils dug up in Africa are creating messy kinks in the iconic straight line of human evolution from knuckle-dragging ape to briefcase-carrying man.(AP Photo/Karel Prinsloo) less

Frederick Kyalo Manthi , Phd, holds the H. erectus complete skull he discovered in 2000 near lake Turkana in Kenya, Wednesday, Aug. 8, 2007 at the National Museum of Kenya in Nairobi. Surprising fossils dug up ... more

Photo: KAREL PRINSLOO

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Anthropologists split on what fossil find means about early humans

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A trove of fossil skulls, teeth and bones more than a million years old and discovered in Africa has opened a new controversy over the lineage of our early human ancestors: Just who descended from whom, and how long ago did they do it?

Scientists have long believed that two species of humanlike creatures were direct forebears of our own Homo sapiens tribe, but the discoverers of the newly described fossils suggest the two species were not directly ancestral at all. Instead, the anthropologists conclude, they lived at the same time in the same African lake region for at least half a million years, and one of them may well have been more apelike than human.

The finds from the Lake Turkana region of Kenya were announced Wednesday by a team of anthropologists headed by Maeve Leakey, the legendary fossil hunter, plus her anthropologist daughter Louise and her principal colleague, Fred Spoor of University College London.

But other anthropologists promptly challenged the team's conclusions.

Such disputes are by no means rare in the contentious field of paleoanthropology, for ancient bones must often be dug from solid rock and are more often than not in bad shape for positive analysis. Their significance is often argued for years.

Still, the explorers keep exploring, the diggers keep digging, and ever since the 1972 discovery of "Lucy," the diminutive 3 million-year-old Ethiopian creature called Australopithecus afarensis - who surely existed somewhere along the complex evolutionary paths toward humanity - the field has been strewn with debate and conjecture.

Reporting in today's issue of the journal Nature, Leakey, Spoor and their colleagues described the upper jawbone of a creature known as Homo habilis (handy man) that dates from about 1.44 million years ago and was found near the Kenyan village of Ileret just northeast of Lake Turkana, very near the Kenya-Ethiopia border.

The other find from the same region was an "exquisitely preserved" skull that the Leakey team identified as Homo erectus (upright man) and dated to about 1.55 million years ago.

Until now anthropologists have held that the ancestral line of early humans went from H. habilis to H. erectus, the direct evolutionary ancestor of all us true Homo sapiens who first emerged on the African scene between 160,000 and 200,000 years ago.

But if the two ancestral Homo species lived side by side, said Leakey in a statement from Nairobi, "their co-existence makes it unlikely that Homo erectus evolved from Homo habilis." In fact, she and her colleagues maintained, both species must have originated much earlier - perhaps between 2 million and 3 million years ago.

"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition," Leakey said.

The skull of Homo erectus is the smallest one found thus far anywhere in the world, according to Spoor, and because other much larger skulls of the same species are already well-known, the size difference indicates that "sexual dimorphism" was a typical characteristic of the line - much more like chimps, where males and females differ greatly in size in contrast to humans.

Tim White, the UC Berkeley paleontologist who has led fossil-hunting teams in the Middle Awash region of Ethiopia for decades and has worked in Kenya too, was highly skeptical of much of the Leakey-Spoor report.

"It seems most likely that they have found a half palate of a large Homo erectus individual and misidentified it as Homo habilis," White told The Chronicle. "Their claim that it represents a geologically late survival of the H. habilis lineage is wholly unconvincing and they will need more than heavily worn, broken and distorted upper teeth to establish that Homo habilis persisted as late as they claim."

David DeGusta, a Stanford paleoanthropologist whose research focuses on the bone structure of early prehuman ancestors, was equally skeptical of the Leaky team's claims.

"I think some scientists will question the species identification of the new fossils, and perhaps the dating as well," he said.

Nor is the Leakey team's claim persuasive that their small Homo erectus skull shows the chimplike characteristics of sexual dimorphism, DeGusta said. "That evidence is already clear in many other Homo erectus fossils," he said. "So even if their identification is correct, they've only added a single juvenile specimen to all the known examples of Homo erectus, and one juvenile fossil isn't going to change our understanding of the typical pattern of differences between males and females" in that species, he said.

"The hypothesis that Homo erectus evolved from Homo habilis is based on hundreds of fossils and their associated dates," DeGusta said. "This new proposal that they existed at the same time is based on two fossils, so I'm going to stick with the old version for now."

At Harvard, G. Philip Rightmire, another specialist on the evolution of Homo erectus who has worked at Dmanisi, a major site in the former Soviet state of Georgia, sided with the Leakey team.

The evidence is "pretty solid," he said, that "this little woman" - the small skull found in Villaret by Maeve Leakey and Spoor - was indeed an example of Homo erectus. And it strongly supports the idea of sexual dimorphism in that species - a key point, he maintained.

There is also "pretty strong evidence" that in Africa, at least, the two species did indeed coexist for thousands of years, Rightmire said.

It's quite possible, he said, that millions of years ago some early versions of Homo habilis moved out of Africa and settled in western Asia - perhaps at the very site where Rightmire hunts fossils with the noted Georgian anthropologist David Lordkipanidze - and then evolved into the Homo erectus lineage that later moved back into Africa to become the true ancestors of Homo sapiens.

"That's only a speculation," Rightmire conceded, "but a nice one."

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